Terminology or glossary of Strabismology

(S. A. Patney: Strabismology Desk Reference, chapter 2, 1996, JKAI Publications, pp 6-17)

Note: In this chapter on glossary or terminology of strabismus and related disorders we have tried to include all or most of the words in common use. In the table given below the terms are followed by their definitions in short. For a short cut to the word please use the underlined capital letters given below by clicking on them.

The Term	Short definition
A
Abduction	Uniocular rotation towards the ear or temporally
Abnormal retinal correspondence	Anomalous relationship of one macula with a nonmacular retinal point. Also referred to as ARC.
Accommodative esotropia (ET)	ET caused by an abnormal demand or supply of accommodation. Hypermetropic glasses reduce it
Acquired squint	Strabismus starting after the first 6 months of life
Adduction	Uniocular rotation towards the nose or nasally
After images	Positive and negative visual impressions following exposure of the macula to a bright light. Used in testing retinal correspondence.
Alternation	Equal ability of each eye to maintain fixation of an object
Alternating heterotropia	Each eye takes turns to deviate and /or fixate an object of attention.
Amblyopia	Defective vision in the absence of an organic lesion. It is not correctable with proper glasses.
Amplitude of motor fusion	The range in degrees or prism diopters through which fusion and binocular vision is maintained despite vertical and horizontal vergence movements.
Angle alpha	Angle formed by the optic axis and visual axis
Angle kappa (Angle lambda)	Angle formed by pupillary axis and visual axis
Angle gamma	Angle formed by optic axis and fixation axis.
Anomalous ret. Correspondence	Same as ARC or abnormal retinal correspondence (see above)
B
Binocular movements	Also called versions (see versions) or yoke movements
Binocular triplopia	Combination of binocular and monocular diplopia
C
Comitance (Concomitance)	When the angle of strabismus is the same in all directions of gaze.
Command movements	Occurring in response to a command, they are a variety of saccadic eye movements.
Compensation (Compensated)	When the heterophoria is well controlled and symptomless with a good range of fusion.
Complete ophthalmoplegia	Paralysis of all intrinsic (internal) and extrinsic (external muscles) of the eye.
Concomitant or Comitant	Strabismus is comitant when its angle is almost the same in all the cardinal directions of gaze.
Confusion	Overlapping of dissimilar images formed on the two maculae because of a deviation of visual axes, being projected in a common direction (straight ahead).
Conjugate movements	Binocular ocular movements or versions taking place in a common direction.
Consecutive strabismus or Consecutive heterotropia	Strabismus consequent upon another type, e.g., exotropia after surgery for esotropia (ET). Or else, ET may become XT (exotropia) with the passage of time.
Constant strabismus or Constant heterotropia	The deviation that is manifest all the time for all the distances either because of a deviation too large to be controlled or because of a weak fusion.
Convergence	A disjunctive or disjugate (vergence) movement where by the two visual axes converge to meet at near fixation-point
Convergence deficiency	Absent convergence in the presence of normal adduction in each eye. It is also called convergence paralysis.
Convergence excess esotropia	Manifest convergent strabismus, only present for near.
Convergence insufficiency	One of the most common causes of eye- strain. The near point of convergence recedes with normal accommodation.
Convergence- insufficiency-exotropia	Divergent strabismus for near and no deviation or exophoria for distance.
Cyclic strabismus	Strabismus appearing at regular intervals, e.g., on alternate days. On other days there is no deviation or a small latent one.
Cyclopean eye (the third eye or the binoculus^*)	The two eyes being strictly conjugate or conjunctive organs, all common visual directions from various retinal points intersect at one point on the principal visual direction. This point is the subjective equivalent of the two eyes^*. Brain receives the sensations coming from the two eyes as if they are from one central eye situated between the 2 eyes.
Cyclophoria	Latent cyclo (torsional) deviation with rotation of the eye around antero-posterior axis causing tilting of the vertical corneal axis towards the nose (incyclophoria) or the ear (excyclophoria).
Cyclorotation	Rotation of the globe (eyeball) around the visual axis, either unilateral (cycloduction) or binocular (cycloversion). It compensates for the tilting of vertical corneal axis caused by postural changes of the body and head.
Cyclotropia	Manifest torsional deviation (see Cyclophoria for details), incyclotropia or excyclotropia.
Cyclovergence	A disjunctive movement whereby the two vertical corneal axes move in opposite directions, one moving towards the left and the other to right. Thus they may diverge (excyclovergence) or converge (incyclovergence).
Cycloversions	Conjugate movements whereby upper tips of vertical corneal axes or the 12 O’clock points of the corneas rotate in the same direction in the two eyes, e.g., to the right. The movement takes place to compensate for a tilting of the head so that the vertical corneal axes remain parallel and upright.
Cyclovertical deviation / strabismus	A combination of vertical and torsional deviations (latent/phoria or manifest/tropia). They are more common than we realize.
D
Decompensation	A loss of constant effortless control of heterophoria. It occurs because the deviation is too large for the fusional forces to control it or because the fusion is too weak. This results in symptoms of ocular discomfort (asthenopia) on using the eyes or an intermittent or constant strabismus.
Degree	The angle of deviation can be measured in degrees or prism diopters. One degree (^o) is the 360^th part of circumference. One prism diopter (PD) is the angle determined by a deviation of 1 cm at 1 meter. Approximately 4^o= 7 PD.
Deorsumduction	The term describes depression or downgaze of one eye (normally both eyes elevate or depress simultaneously).
Deorsumvergence	An eye is deorsumvergent when it is lower than the other eye. A better word is hypodeviation.
Deorsumversion	Depression of both eyes
Depression	Downward movement of one/both eyes
Dextrocycloversion	Tilting of both vertical corneal axes to right, usually a result of tilting of head to left
Dextrodepression	Movement of both eyes to down and right
Dextroelevation	Movement of both eyes to up and right
Dextroversion	Movement of both eyes towards right
Diagnostic directions of gaze	Also known as cardinal directions or diagnostic positions of gaze. There are 9 directions in all: Up & right, upgaze, up & left, right, primary position, left, down & right, direct down (downgaze) and down & left.
Diplopia (double vision)	Diplopia is the result of simultaneous stimulation of noncorresponding points of the two retinas. The noncorres- ponding or disparate retinal points project the image in different directions or locations in space resulting in double vision. This is called binocular diplopia.
Disjunctive (disjugate) movements	Also called vergence movements during which the two eyes move in two different directions. In convergence (the corneas moving towards each other) and divergence (corneas moving in opposite directions) one cornea moves to right while the other moves to left. The purpose is to maintain fusion while the object of attention moves nearer to or away from the eyes.
Divergence	It is a disjunctive movement during which both eyes move temporally, away from each other.
Divergence excess exotropia	The divergent deviation that is much greater for distance than it is for near.
Divergence insufficiency	There is intermittent or constant esotropia for fixation at distance and binocular fixation for near with good fusional range. If esodeviation is present constantly it is greater for distance. It is comitant (equal in all directions) and movements are normal. No neurological deficit is found unlike in divergence paralysis
Divergence paralysis	No defect of abduction, only of divergence (unlike that in VI CN palsy). Sudden onset of diplopia for distance with esotropia. Usually associated with some neurological problem, e.g., head injury, intracranial inflammations or neoplasms.
Ductions	A word used to describe uniocular movements. Turning of the eye temporally is abduction, turning it nasally is adduction, supraduction is the same as elevation, infraduction is depression and cycloduction is turning the eye around the antero-posterior axis.
E
Eccentric fixation	In long standing cases of strabismus the deviating eye does not fixate centrally even when the good eye is occluded. When it is made to fixate it does so with an eccentric point on the retina (not with macula). It is always associated with amblyopia and abnormal retinal correspondence. Thus even though the image is forming on an extrafoveal point the image is projected centrally.
Eccentric viewing	An eccentric point of retina is used for fixation (usually because of pathology in the macula) but it retains its normal projection and retinal correspondence. Thus an image forming on it will be localized not at the point of fixation but elsewhere, e.g., a nasal point will project the image temporally.
Elevation
Esohyperdeviation	Horizontal deviations are often associated with hyperdeviations. Here the esodeviation is accompanied by hyperdeviation. It may be a part of A-V pattern.
Esophoria (E)	A latent deviation of the eyes inwards. It is controlled by the forces of fusion. Diagnosed by cover test and confirmed by instruments like prism bars, synoptophore and others used for measuring the angle of deviation.
Esotropia (ET)	Manifest convergent deviation
Excycloduction	Movement of the globe (eyeball) around the anteroposterior axis so that the 12 O’clock position of the cornea at the upper end of vertical corneal axis rotates outwards
Excyclophoria	Latent rotational or torsional deviation of the eye in which the 12 O’clock point of the cornea rotates outwards. The word is often used for excyclotropia.
Excyclotropia	Manifest excyclodeviation of the eye (see excyclophoria)
Excyclovergence	A disjunctive movement of the eyes (like divergence) in which the 12 O’clock points of both the corneas rotate outwards.
Exohyperdeviation	Divergent deviation associated with hyperdeviation. It may be a part of A-V pattern.
Exophoria (X)	Latent divergent deviation of the visual axis. The deviation is kept under control by the forces of fusion
Exotropia (XT)	Manifest divergent deviation of the visual axis
External ophthalmoplegia	Paralysis of the extraocular muscles (EOM)
F
False macula / Pseudo-macula	A term used sparingly now to designate the extrafoveal retinal point in the deviating eye, that functions with macula of fixing eye in cases of anomalous retinal correspondence.
False projection	Alternative names: false orientation / past-pointing. An anomaly of egocentric localization in not too old paralytic strabismus. When the patient is asked to point to an object placed in the field of action of paralytic muscle, with other eye occluded, the patient’s finger overshoots the mark.
Field of fixation	It is the area in which central fixation is possible. Only the eye is moved while the head stays fixed.
*Fixation	It is a sensory-motor action by which the eye is aligned to the object of attention / regard. Under normal conditions it is foveal.
Fixation disparity	A physiological variant of binocular vision in which there is a minute image displacement by a few minutes of arc of angle within the Panum’s area, while fusion is maintained. Usually accompanied by micro-esotropia with full binocular functions.
Following move-ments	Alternative name: Smooth pursuit. Movements of eyes made while following a moving object.
Fusion (Sensory)	Unification of visual excitations from corresponding retinal points leading to images, into a single image. Simply stated it is the ability of the brain to bring together two sensations resulting into one single perception. When the images of an object fall on correspond- ing retinal points (e.g., foveas), they are seen as one single image.
Fusional move-ments	They represent the motor aspect of fusion. The vergence movements, e.g., convergence, bring about and maintain alignment of the two eyes and thus maintain fusion.
G
Grades of binocular vision	Relevant only relation to the major amblyoscope. I grade or simultaneous perception (SP) is the ability to perceive two dissimilar images, e.g., man and gate, (one formed in each eye) are seen superimposed. In II grade or fusion (F) two similar but in-complete images are seen as one complete image. In III grade or stereoscopic vision (SV) two slightly different images are seen in third dimension or depth.
H
Haploscopy	It is the use of Haploscopic devices. It is vital for investigating the sensory-motor co-ordination of the eyes. Examples of these devices are major amblyoscopes, stereoscopes and Phase difference haploscopes etc.
Head tilting test (Bielschowsky’s)	It is used to distinguish between a SO palsy and a contralateral SR palsy. It is positive in SO palsy. When the head is tilted to the side of SO palsy, say right, right eye will be pulled up by the un-opposed right SR muscle when the two muscles are trying to cause incycloduction of the right eye.
Heterophoria	A common term applied to a latent deviation in horizontal, vertical or Cyclotorsional direction. The different types are Esophoria, Exophoria, Hyperphoria, Hypophoria and Cyclophoria (Incyclo and Excyclophoria).
Heterotropia	It is a manifest deviation of different types: Esotropia, Exotropia, Hypertropia, Hypotropia or Cyclotropia.
Hyperphoria	It is a latent upward deviation and is right, left or alternating hyperphoria.
I
Incomitance	It was also known as noncomitant in the past. An incomitant deviation is the opposite of comitant. It is usually due to an extraocular palsy. The angle of deviation changes in different directions of gaze and when either eye fixates
Incycloduction or intorsion	A less appropriate word is a movement of one eye around its anteroposterior axis whereby the upper tip or the 12 O’clock position of vertical corneal axis moves inwards or towards the nose.
Incyclophoria	It is a latent deviation in which the upper tip or the 12 O’clock position of the vertical corneal axis tilts inwards or nasally, when the eyes are dissociated and effect of fusion is removed temporarily as by a cover test.
Incyclotropia	It is a manifest deviation in which the upper tip or the 12 O’clock position of the vertical corneal axis tilts inwards or nasally.
Incyclovergence	It is the tilting of the upper tip of each vertical corneal axis inwards or nasally whereby the 12 o’clock of the right cornea tilts towards the left and that of the left eye towards the right. Thus they tilt towards each other.
Infantile strabismus	The strabismus that starts within the first six months of life.
L
Left deorsumvergence	It is also called right sursumvergence. In this condition the left eye moves down and the right moves up.
Left sursumvergence	It is also called right deorsumvergence. In this condition the left eye moves up and the right moves down.
Levocycloversion	It is a binocular movement in which the upper ends of both the vertical corneal axes (VCA) tilt towards the left. Thus there is incycloduction in right eye and excycloduction in left eye. It normally occurs when head is tilted to right.
Levodepression	It is a binocular movement to down and left.
Levoelevation	It is a binocular movement to up and left
Levoversion	It is a binocular movement to left
M
Mechanical strabismus	Also called Restrictive / Structural. Contracture of a muscle affects function of other muscles particularly its antagonist.
N
Negative vertical divergence	It is the same as left hyperdeviation or right hypodeviation
Nonaccommodational esotropia	Accommodation and its defects do not have much of a role to play in the causation of this type of strabismus.
O
Occlusion amblyopia	It is the amblyopia resulting from occlusion of the sound eye to improve vision in the amblyopic other eye. The younger the child / infant, the shorter is the period of occlusion required to cause amblyopia. The vision should therefore be checked frequently to ensure prevention of occlusion amblyopia.
Organic amblyopia	Amblyopia superimposed upon an organic defect of the retina, optic nerve and media
Oculomotor palsy	It results from involvement of 3^rd cranial nerve leading to paralysis of muscles supplied by it, i.e., medial rectus, superior rectus, inferior rectus inferior oblique and levator palpebrae superioris.
Ophthalmoplegia	Paralysis of the extraocular and /or intraocular muscles of the eye
Orthophoria, also called orthotropia	An ideal condition in which the visual axes remain aligned with the object of attention (fixation) with or without the influence of fusion. Cover test reveals no deviation even under cover
Orthoptics	In the widest sense, it aims to re-establish comfortable (binocular) and equal vision. In the practical sense however, it is confined to investigation and non-surgical treatment of the problems of binocular vision and ocular motility.
P
Panum's area of binocular single vision	For each retinal point there is a small elliptical zone in the contralateral retina, that surrounds its corresponding point. The stimulation of this point along with its contralateral corresponding point results in the sensation of a single image. Added to that is a sense of depth perception or third dimension.
Palsy or paralysis	It is a complete abolition of the action of a muscle or a group of muscles.
Paresis (partial paralysis)	It is only an impairment of the action of a muscle or a group of muscles (not a total loss of function).
Pleoptics (pleos=full; optikos=ab-out eyes)	A branch / associate of orthoptics, which is concerned with treatment of amblyopia, particularly that with eccentric fixation. The term was coined by Bangerter.
Positive vertical divergence	Also called right hyperdeviation.
Primary heterotropia	Arises from unknown causes (idiopathic heterotropia)
Primary position	When eyes look straight ahead with head and body held erect. It is the basic position at which the examination for ocular muscles' imbalance starts.
Principal visual direction (PVD)	It is the principal direction of reference for oculocentric localization. This point when fixed, gives the impression of being seen straight ahead. Normally it is related to fovea, the point with maximum visual acuity.
Prism diopter / PD	1 prism diopter is the angle determined by a deviation of 1 cm at one meter. 4 degrees = 7 PD
Proprioceptive impulses	They come from muscles or from labyrinth and carry information to the brain about the position and the movement of the body and therefore of the eyeball (globe), if these impulses are coming from the extraocular muscles.
R
Rapid eye movements (RPM)	One of the group of fast saccadic eye movements, they are random eye movements that occur during sleep.
Recession (retroplacement or retropositioning	A surgical procedure in which the muscle is cut at its insertion and reattached further back to weaken the effect of its contraction, e.g., medial rectus recession for esotropia.
Restrictive strabismus	Same as mechanical strabismus. Please see mechanical strabismus.
Retinal Rivalry	When two dissimilar images fall on the corresponding areas of the retina, there is confusion due to their superimposition and there is a conflict between the two retinas. The brain therefore suppresses one of the images. If both eyes have equal dominance there is alternating suppression of these images, e.g., while wearing red glass in front of one eye and green in front of the other, a plane white surface will appear red and green alternately.
Right deorsum- vergence	It is the same condition as left sursumvergence, or negative vertical divergence. There is elevation of left eye, relative to right eye.
Right sursumvergence	Also known as left deorsumvergence or positive vertical divergence.
S
Saccades (Also known as left deorsumvergence)	The purpose of these eye movements, a kind of versions, is to place the image on the fovea and to keep it there as long as required. When optical stimuli from an object in the periphery of the visual field, which attracts attention, reach the retinal periphery they elicit saccades. Other examples of fast saccades are random command movements, movements during fast phase of nystagmus, rapid eye movements (REM) during sleep and correcting saccades during fast pursuit movements.
Secondary heterotropia	A manifest squint, which is secondary to either defective vision in one eye (sensory heterotropia) or occurs due to overcorrection of a strabismus, for instance, a previously esotropic eye becoming exotropic (consecutive heterotropia).
Sensory hetero-tropia	Please see secondary heterotropia above.
Simultaneous perception	Worth divided binocular vision (BV) into 3 grades, which are mainly relevant to examination on major amblyoscope. First grade is simultaneous perception (SP), which is the ability of the brain to perceive two dissimilar images, one formed in each eye, simultaneously. It is tested on synoptophore by superimposing the SP targets like the joker and the gate. For details see grades of BV in terminology.
Smooth pursuit or Following movements	They are made while following an object. They are a kind of versions. Relatively slow movements, they are brought into play while tracking a moving object.
Squint	A deviation of visual axis from its fixing position. Usually the word is used for a manifest deviation.
Stereopsis	The perception of depth or third dimension. The sensation is caused by the images falling on the fovea of one eye and its Panum's area in the other.
Strabismic amblyopia	Depressed vision uncorrectable by corrective glasses. It results from squint. It is caused by an active suppression of fovea and a passive disuse (disputable).
Strabismus	See squint above.
Strabismus deorsoadductorius	Is the name given to a downshoot in adduction. Usually it is due to superior oblique overaction.
Strabismus Fixus	A rare condition in which one or both eyes are fixed or anchored in a certain position, usually adduction. Cases of vertical strabismus fixus are still rarer. The eye can not be moved in any direction. The condition is thought to be due to congenital fibrosis of extraocular muscles.
Strabismus sursoadductorius	Is a name given to upshoot of the eye in adduction. It is usually due to an overaction of inferior oblique.
Suppression	Is an active central inhibition of image responsible for diplopia and confusion, which result from a manifest deviation. It is usually found in children and is limited to binocular vision. If the fixating eye is covered, suppression in the deviating eye disappears.
Suppression scotoma	To avoid diplopia and confusion resulting from a deviation, suppre-ssion occurs in the fovea of the deviating eye and that area on the periphery where the image of the object of attention falls (fixation point scotoma). In cases of alternating deviations suppression scotomas are found alternately in each eye ( the nonfixing eye).
Sursumduction	Elevation of one eye
Sursumvergence	Disjugate movement in which one eye moves up and the other down (see right and left sursumvergence
Sursumversion	Simultaneous elevation of both eyes
T
Triplopia	Triple vision. The patient complains of seeing three images of one object, two with one eye and one with the other.
Tropia	Short for heterotropia (manifest strabismus)
U
Unilateral heterotropia	Manifest strabismus in one eye
V
Vergence	Disjunctive / disjugate movements. The two eyes move in opposite directions. (One to the right and the other to the left) e.g., convergence and divergence.
Versions	Simultaneous movements of the two eyes in the same direction, e.g., dextroversion
Vertical angle Kappa	It simulates a vertical deviation and is usually caused by a vertical displacement of macula (macular ectopia)
Vertical vergence movements	When a prism is interposed in front of the eye, the cornea moves towards its apex. , e.g., a base up prism opposite OD will lead to movement of the right eye downwards. It is a vertical disjugate movement as the OD moves away from OS..
V esotropia (ET)	The angle of ET increases in downgaze and decreases in upgaze.
V exotropia (XT)	XT increases in upgaze and decreases in downgaze.
Vision deprivation	It is a lack of stimulation of retinal cells, either due to a squint or due to an obstruction to vision caused by occlusion or opacities in the media.
Visual axis	A straight line joining the fovea with object of attention. It passes through nodal point. Other names are: line of, vision / gaze / regard / principle visual direction
X
X pattern	XT minimum in PP ; ET maximum in PP
Y
Y ET	Fusion in upgaze, ET in PP & downgaze
Y XT	XT in upgaze only